Exploring Root Zone Moisture Dynamics

March 3, 2023 steven

The root system is the powerhouse of the plant, responsible for absorbing water and nutrients. It must not only maintain its own growth and development, but also provide essential supplies for fruit, flowers, and foliage. Root zones are difficult to monitor because they are enclosed, dark, and are often overlooked, however, they are essential for driving yields and overall plant vigor and, even in hydroponics, need a little consideration now and then.

For substrate-grown crops, moisture dynamics in the root zone can be a useful tool to steer crop development, ensure high levels of oxygenation, and act as a reservoir of water and nutrients between irrigations. Fortunately, these days we have such a wealth of high-quality hydroponic substrates to choose from that crops can be matched to a growing medium and moisture levels precisely controlled.

Why are Moisture Dynamics Important?

Within the root zone of a substrate-based hydroponic system, there should exist a gradient in moisture levels from the top to the base of the growing container/bag/bed/slab. This is due to the effect of gravity but is modified by the size and shape of the growing container and the physical properties of the substrate. This means there is a reserve of moisture in the lower layers of the substrate and a higher percentage of larger air-filled pores towards the surface. Both are essential for plant growth and development, however, matching these properties to plant species and growing conditions is worth investing a little time into.

A water-holding capacity of 45-65 percent is considered optimum for most soilless growing media in hydroponics. Water-holding capacity is the portion of container pore space that retains water after drainage is complete and this can be easily measured: A container filled with the substrate is fully saturated with water and then permitted to drain. Once draining is complete the damp medium is weighed, dried at 230°F (until all moisture has been removed, then re-weighed). The difference between the wet and dry weight is the water holding capacity of that substrate. This can be a particularly useful exercise for determining the water-holding capacity of substrates that are a blend of different materials to determine if they fall within the optimal range.

Different Crops, Different Tolerances

_{Crops with large leaves, grown under warm conditions, put extra pressure on the dynamics of the root zone.}

Some plant species are highly prone to problems with overwatering, while others are more tolerant. Strawberries are one plant that has no tolerance for a saturated substrate and crop losses have resulted from overwatering the crown and the root rot that often follows. Many cacti and succulents will also rot when overirrigated and prefer a coarse, free-draining substrate such as perlite or coarse sand.

Other plants, more notably those that are grown under warm, high-light conditions, have large leaves and a rapid rate of growth that are better suited to highly moisture retentive mediums that will hold sufficient water between irrigations. Cucumbers, tomatoes, squash, and similar crops perform well in a medium that has a high water-holding capacity and also a good rate of air-filled porosity to provide oxygenation. These often include substrates such as coconut fiber, which has a blend of both moisture retentive ‘fines’ or finer coir particles, mixed with longer, coarser fibers that retain the open structure the medium and provide a suitable level of aeration and drainage.

Substrates and Root Zone Dynamics

_{Checking that the root system is healthy and spreading through the substrate is a good idea.}

Some manufactured hydroponic substrates such as stonewool slabs or blocks, and even some coco fiber products, can have physical properties specifically designed for certain crops. Stonewool slabs have differences not only in overall moisture and air-filled porosity levels, but also carefully calculated moisture gradients between the top and base of the slab. Many of these specialized products are aimed at different crops, growing climates and uses, and help provide the optimum levels of moisture in the root zone.

Growing conditions also play a role in root zone moisture dynamics. The rate of transpiration, temperature, and humidity should be taken into account when choosing a substrate. Crops growing under warm, high-light, low-humidity conditions requires frequent irrigation and benefit from a moisture retentive medium that helps prevent drying out of the root zone and gives more of a safety buffer should failures with pumps or the power supply occur. Under cool conditions with slower growth and small plants, substrates that are free draining and retain lower levels of moisture assist with prevention of oversaturation. These types of free draining open substrates are also more forgiving of the application of high levels of nutrient solution that may be needed for plant nutrition, but at the same time not contributing to a water-soaked root zone when growth and transpiration rates are low.

Root Zone Dynamics and Oversaturation

_{Strawberries are a crop that has little tolerance for oversaturated root zones.}

When nutrient solution is irrigated onto a substrate it displaces air in the open pores of the material. When draining subsequently occurs, more air is drawn down into the root zone. If overirrigation is occurring, the air-filled pores remain saturated for too long and the plant has less access to the oxygen contained in the air. Oxygen is essential for root system respiration and healthy functioning and considerably more O₂ is present in air than can be dissolved into the nutrient solution. Plants exhibit a strategy termed ‘oxytropism’ where roots will avoid growing into oxygen-deprived areas such as overwatered hydroponic substrates and stagnant nutrient solutions. This is most often seen inside the base of growing containers or slabs of substrate as areas devoid of any root growth or in small, thin, brownish roots that have died back due to suffocation and oversaturation.

Other symptoms of an overly wet substrate and water logging are chlorosis (yellowing) of the new foliage, older leaves may yellow and fall from the plant, flower and fruitlet drop are also common as are outbreaks of root rot pathogens such as Pythium. Another symptom of extreme overwatering is epinasty where excess moisture in the root zone causes ethylene gas to build up in the plant resulting in severe bending downwards of the leaves. This is often mistaken for wilting, however, growers should always check to determine if any wilting is actually due to an overly dry root zone or epinasty because of water logging.

Root Zone Moisture Dynamics and Crop Quality

_{Tomato fruit splitting can have a number of causes, though large moisture fluctuations in the root zone are a common cause.}

Choosing a hydroponic substrate that matches the crop and climate is important not only for root health and general plant growth, but also crop quality. Common mistakes with root zone moisture dynamics not only risk root rot, but large fluctuations in moisture levels can predispose some crops to fruiting problems such as fruit surface crazing or splitting in tomatoes, capsicum and chillies, and fruit cracking in melons and cucumbers. Selection of the right substrate, combined with a suitable irrigation program to maintain an even moisture level in the root zone can help prevent these issues in sensitive crops.

Another method of quality improvement that involves moisture dynamics in the root zone is deficit irrigation — this is the careful restriction of irrigation so that the plants experience slight moisture stress. This has a number of physiological effects on plants but is largely practiced to improve the compositional quality of fruits such as tomatoes or grapes. By restricting water uptake, more sugars and other compounds concentrate in fruit tissue, thus giving an improved flavor profile. In crops such as herbs, this can also result in higher levels of volatiles which contribute the distinctive flavor and aroma of the harvested product. The issue with using deficit irrigation in hydroponics is that it requires a high degree of control and grower skill to ensure plants are only mildly stressed and not permanently damaged with no significant loss in yield.

Apart from improvements in fruit quality, deficit irrigation is also a technique used to help ‘steer’ crops away from being overly vegetative and towards a more generative (flowering and fruiting) balance. Deficit irrigation may include reducing the volume of nutrient solution applied at each irrigation, allowing more time between irrigations and allowing the media to dry slightly overnight by restricting early morning and evening irrigations. The type of deficit irrigation to force plants into generative growth must be used with caution as excessive moisture fluctuations in the root zone can lead to an increase in fruit problems such as blossom end rot and fruit splitting under certain growing conditions.

Root zone moisture dynamics are part of hydroponic production — root zones need as much monitoring and attention as the aerial portions of the plant to ensure water and oxygen levels are sufficient and not contributing to root dysfunction. Substrate selection and matching to the crop and climate as well as careful control over irrigation are all part of the delicate balance of the root zone.

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